Navigation system, route retrieval server and mobile terminal device, and route guiding method

A mobile terminal device has a display for indicating guidance of a route retrieved by a route retriever and a GPS receiver for detecting the present location. A route retrieval server has a database for storing the present location of each bus running on bus lines and an alternative route guider for evaluating alternative bus route to provide guidance. When the guided route includes a walking route, the route retriever references the database based on the present location of the mobile terminal device to identify an alternative route that includes a bus. The alternative route guider compares the alternative route and guided route. When the alternative route is more favorable than the guided route, the alternative route guider sends alternative route information indicating presence of the alternative route to the mobile terminal device, displays the guided route on a display, and indicates the presence of the alternative route.